Answer
$126x^5$
Work Step by Step
According to the Binomial Theorem we can obtain the $r+1$th term of the expansion of the binomial $(x+y)^n$ by the formula $_nC_rx^{n-r}y^r$.
Hence here it is: (by plugging in $n=9,r=4$ and $x,y=-1$: $_9C_4(x)^{9-4}(-1)^4=15\cdot x^44y^2=126x^5\cdot1=126x^5$
